1. Technical Field
This invention relates generally to encoding pictorial imagery for reproduction on binary display and/or printing systems, and more particularly to increasing the number of discernible gray levels in halftone reproduction.
2. Background Art
Representation of the intensity, i.e., the gray level, of a color by binary displays and printers has been the object of a variety of algorithms. Binary displays and printers are capable of making a mark, usually in the form of a dot, of a given, uniform size and at a specified resolution in marks per unit length, typically dots per inch. It has been common to place the marks according to a variety of geometrical patterns such that a group of marks when seen by the eye gives a rendition of an intermediate color tone between the color of the background (usually white paper stock) and total coverage, or solid density.
Continuous tone images contain an apparent continuum of gray levels. Some scenes, when viewed by humans, may require more than 256 discrete gray levels to give the appearance of a continuum of gray levels from one shade to another.
As an approximation to continuous tone images, pictorial imagery is represented via halftone technologies. In order to record or display a halftone image with a scanning system, one picture element on the recording or display surface consists of a j.times.k matrix of sub-elements where j and k are positive integers. A halftone image is reproduced by printing the respective sub-elements or leaving them blank. That is, by suitably distributing the printed marks.
Halftone image processing algorithms are evaluated in part, by their capability of delivering a complete gray scale at normal viewing distances. The capability of a particular process to reproduce high frequency renditions (fine detail) with high contrast modulation makes that procedure superior to one which reproduces such fine detail with lesser or no output contrast.
Another measure of image processing algorithm merit is the tendency to produce visual details in the output image that are not part of the original image, but are the result of the image processing algorithm. Such details are called artifacts, and include moire patterns, false contours, and false textures. Moire patterns are false details created most often by the beating between two relatively high frequency processes resulting in a signal whose spacial frequency is low enough to be seen by the viewer. False contours are the result of gray scale quantization steps which are sufficiently large to create a visible contour when the input image is truly a smooth, gradual variation from one to the other. False textures are artificial changes in the image texture which occur when input gray levels vary slowly and smoothly and the output generates an artificial boundary between the textural patterns for one gray level and the textural patterns for the next gray level.
Briefly, several of the commonly used processing algorithms include fixed level thresholding, adaptive thresholding, orthographic tone scale fonts, and electronic screening. The present invention is concerned with the latter, electronic screening.
FIG. 1 shows a schematic view of the electronic screening process. Signal X.sub.i represents the lightness or gray level information at a sampling point i of an image. Input signal X.sub.i of sample image picture elements is compared with a series of threshold values C.sub.i selected in sequential order from a two-dimensional matrix defined to be the halftone cell threshold set, and a print/no-print decision is made. The series of threshold values and their arrangement within the threshold set determine the gray scale range, the frequency, angle, and other properties of the halftone pictorial image. Each threshold level C.sub.i is determined by a comparison j.times.k matrix. When the input signal X.sub.i exceeds the threshold level C.sub.i, the corresponding sub-element is determined to have a print level or logic level "ONE". By comparing the input signal X.sub.i with the threshold levels, j.times.k output signals O.sub.i are produced. A density pattern consisting of a combination of j.times.k sub-elements is obtained by dividing each picture element into j.times.k sub-elements and systematically printing them or leaving them blank.
FIG. 2 shows a typical two dimensional matrix halftone cell for electronic halftoning with 18 possible gray levels, used as a 45.degree. angular screen. When the cell is repeated horizontally and vertically, it creates the entire screen function. FIG. 3 shows the possible "nonwhite" halftone sub-elements which may be generated by the screen function of FIG. 2.
A problem exists with the number of density levels attainable with a limited resolution and acceptable screen frequency. A 94.5 lines per inch, 45.degree. screen using a 400 dpi system results in nineteen level halftoning, including white. Nineteen levels is not generally sufficient; more gradations being preferred. One way to get more gray levels is to reduce the number of lines per inch, but this decreases the screen frequency to a visible level.
Various screen functions have been proposed for electronic screening to minimize the number of gray levels required to manifest acceptable pictorial imagery. These existing types of screen functions, also referred to as "threshold value matrices," are roughly divided into the following two groups: (1) those in which sub-elements grow around the center core and (2) those in which the spatial frequency of the sub-elements is made to be as high as possible.
Group-1 screen functions are generally known as "fattening" or "dot concentration" type functions. FIG. 4 is a 4.times.4 group-1 matrix. As shown in FIG. 5, sixteen gray levels (plus all white) are obtained by sequentially increasing the number of sub-elements which are printed in black.
Group-2 screen functions are known generally as "dot dispersion" type. The best known dot dispersion type screen functions were developed by Bayer, Lippel, and Jarvis. FIG. 6 is typical a 4.times.4 group-2 matrix. As shown in FIG. 7, sixteen gray levels (plus all white) are obtained by sequentially increasing the number of sub-elements which are printed.